15 < w < 200 (radisec) for the direct coupler. The lower limit common to both devices was probably caused by the thermal convection whose relative effect increases when the velocity of rotation decreases. The upper limit was mainly because of inevitable vibrations which increase with the velocity of rotation and are amplified by the unusual length of the electrode and elastic properties of the glass. The velocity interval in which the electrodes satisfactorily followed Levich’s equation was sufficiently great to permit useful applications of the device for the elucidation of electrode processes. Under optimum conditions ( w = 40-60 radisec), a linearity within 1 for current /concentration curves was readily attained. This certainly qualifies the rotating disk electrode as one of the best quantitative tools for fused salts studies. Under the reported optimum conditions the diffusion coefficient and the temperature coefficient for the NO*- were calculated : [NO;]
; MCLE/1030g.MELT
D N O~ (510 OK) = 5.25
(RAD/SEC)
Figure 4. Graphical verification of the Levich’s equation o Points obtained with the mount of Figure 1 e Points obtained with the mount of Figure 2,1 sodium-potassium nitrate eutectic melt under argon at 510 OK. The limiting currents obtained from a series of such curves ( w = 63 rad/sec) are given in Figure 4,A as a function of the nitrite concentration. A plot of the limiting currents obtained for a given concentration ([NOz-] = 2.45 lO-3m) as a function of w1/2 is shown in Figure 4,B. The plots of Figure 4,A and 4,B, representative of a series of experiments, demonstrate the good agreement of our experimental results with Levich’s theory. The deviation from linearity was estimated to be better than 2% with both designs described whenever 15 < w < 90 (radisec) for the magnetic coupler and whenever
E (500
- 560
O K )
=
+ 0.1 X
10-6 cmZ/sec
-5.4 Kcal/mole
(4) (5)
Because of the characteristics of the material used, the device described herein cannot be used above 650 OK. However, if a classical, water cooled brass head is substituted for the glass cover, the apparatus can then be used up to 1000 OK-Le. the softening temperature of the glass. ACKNOWLEDGMENT
The author expresses his appreciation to Joseph Jordan and his research group for helpful discussion. RECEIVED for review September 3, 1968. Accepted December 31, 1968. Investigation supported by U S . Atomic Energy Commission, Report No. NYO-2133-46.
Gas Chromatographic Separation of Chlorosilanes, Methylchlorosilanes, and Associated Siloxanes K. Ray Burson Texas Instruments Inc., Mail Station 913, P . 0. Box 5012, Dallas, Texas 75222
Charles T . Kenner Department of Chemistry, Southern Methodist University, Dallas, Texas 75222
SEVERAL methods have been reported for gas chromatographic separation of chlorosilicon compounds. Turkel’taub et al. ( I ) studied the effect of the nature and amount of the stationary phase, the solid support, the flow rate of the carrier gas and the temperature of the column during the investigation of the separation of hydrogen chloride, silicon tetrachloride, and the methylchlorosilanes. They used a column containing nitrobenzene preceded by another identical column to offset bleeding. Oiwa ( 2 ) separated the methylchlorosilanes by using two columns in series, one containing tritolylphosphate and the other dioctylphthalate as the liquid
(1) N. M. Turkel’taub, et al., Ref. Zhurl. Khim., 23, Abs. No. 23D135 (1961); Anal. Abst., 3294 (1962). (2) T. J. Oiwa, Chem. Soc. Japan, Pure Chem. Sect., 84, 409 (1963); Anal. Abst., 5230 (1965). 870
ANALYTICAL CHEMISTRY
phases. Bersadschi et al. (3) separated trichlorosilane and silicon tetrachloride mixtures in carbon tetrachloride using transformer oil activated by glycerol as the liquid phase. Lengyel et al. ( 4 ) and Palamarchuk et al. ( 5 ) investigated the separation of methylchlorosilanes using different liquid stationary phases and found those with the highest dipole moment to be the most effective. Popov et al. (6) studied the separation of chlorosilanes, methyl chlorosilanes, and phosphorus (3) D. Bersadschi, V. Stefan, and Petroianu, Rev. Chem. (Bucharest), 15, 224 (1964); Ana/.Abst., 3808 (1965). (4) B. Lengyel, G. Garzo, and T. Szekely, Acta. Chim. Acad. Sci. Hung., 37,37 (1963); C. A , , 5 9 : 4541c (1963). ( 5 ) N. A. Palamarchuk, et al., Inst. Geochim. i Analit. Khim. 13, 277 (1963); C. A., 5 9 : 6994 (1963). (6) A. N. Popov, V. M. Gorbacher, and E. I. Torgova, Ser. Khim. Nauk, 3, 17 (1966); C . A , , 67: 28950s (1967).
Table I. Column Data
Stationary phase Dimethylphthalate Diethylphthalate Dibutylphthalate Dipropyltetrachlorophthalate
Dinonylphthalate SF-96 Silicone oil DC-704 Silicone oil QF-1 Silicone oil XE-60 Silicone gum DC-710 Silicone oil LSX-3-0295 Silicone gum ~
Concn. (wt %)
Particle size
30 30 30 20 30 10 20 20 10 20 10
60180 60180
60180 60180 30150 60180 60180 60180 60/80 60180 60180
Column dimension Length Diameter (feet) (inches) 12 12 6 6 6 12 6 6 18 6 12
31~6
V16 3i16
VI 6 '14 6
'14
V16 3iI6
3i1 si16
HETP
(cm) 1.37 1.48 1.85 1.55 1.81 1.65 1.20 1.12 1.17 1.85 1.25
Conditioning temp ( "C)
Initial ("C)
50 75 100 125 175 300 250 250 215 300 250
25 25 25 25 25 25 25 25 25 25 25
Program data Final Rate (Tjmin) ( "C) 50 50 50 75 100 250 200 200 200 200 200
5 5 5 5 5 10 10 5 5 5 5
~~
Stationary phase Dimethylphthalate Diethylphthalate Dibutylphthalate Dinonylphthalate
Table 11. Retention Times for Components of Silane Mixtures Retention time (minutes) Silicon TrimethylMethyltriTrichlorosilane tetrachloride chlorosilane chlorosilane
Dipropyltetrachlorophthalate SF-96 Silicone oil DC-704 Silicone oil QF-1 Trifluoropropyl Silicone oil XE-60 Cyanoethylmethyl Silicone oil DC-710 Phenyl Silicone oil DC-LSX-3-0295 Trifluoropropyl silicone gum
8.5 7.9 5.7 5.5 3.0 3.6 3.6 2.2 7.3 2.8
10.2 9.5 7.1 9.2 3.9 7.8 5.5
5.8
Dimethyldichlorosilane
4 3
15.0 13.8 14.1 13.6 4.8 6.8 4.6 4.3 12.3 4.5
20.1 18.8 17.9 16.4 6.7 10.8 9.7 5.2 15.0 6.7
23.3 20.9 20.8 17.3 6.8 10.8 9.7 6.0 17.0 6.7
7.8
10.2
11.5
13.0
3.1 8.8
chlorides and found polar compounds were retained more strongly by the more polar stationary phases. Nonpolar compounds-e.g., silicon tetrachloride-were not affected by the polarity of the stationary phase. These investigations have dealt primarily with the separation of the chlorosilanes and methylchlorosilanes. In many samples of these compounds, siloxanes produced by hydrolysis are also present but are not eluted in a reasonable length of time under isothermal conditions. Temperature programming is necessary for elution of the siloxanes, but none of the stationary phases studied in the previous investigations can be used with temperature programming because of their high volatility at elevated temperature. The purpose of this study was to evaluate the stationary phases that could be used with temperature programming for the separation of mixtures of chlorosilanes, methylchlorosilanes, and their associated siloxanes, and to develop a quantitative method for the analysis of such mixtures. EXPERIMENTAL
Apparatus. The gas chromatographic measurements were made on an F & M Model 700 dual column, programmedtemperature instrument equipped with a thermal conductivity detector with tungsten-rhenium filaments. The detector was held at 230 "C and the injection port at 180 "C. Helium was used as the carrier in all cases at a flow rate which gave the lowest HETP for the particular column being evaluated. Stationary Phases. The following liquids or gums were used as stationary phases in this investigation: dimethylphthalate and dibutylphthalate (Fisher Scientific); dinonylphthalate (J. T. Baker); diethylphthalate (City Chemical);
dipropyltetrachlorophthalate (Eastman); QF-1 silicone oil and XE-60 silicone gum (Applied Science); LSX-3-0295 silicone gum (Analabs); DC-710 and DC-704 silicone oils (Dow-Corning) ; SF-96 silicone oil (General Electric). Chlorosilanes. Trichlorosilane, silicon tetrachloride, trimethylchlorosilane, methyltrichlorosilane, and dimethyldichlorosilane were purchased from Union Carbide and redistilled before using. Columns. Columns were packed with the stationary phases and utilized under the conditions listed in Table I with acidwashed Chromosorb-P as the inert support. The packed columns were installed in the gas chromatograph and conditioned for 4 hours at the maximum operating temperature of the liquid phase. After conditioning, the column temperature was lowered and its efficiency determined with hexane by the method of Glueckauf (7) using the equation
8 (x/y')* (1) where n is the number of theoretical plates, x is the retention time from injection to apex of peak measured in minutes or millimeters of chart paper, and y' is the width of the peak at its heightle or 0.368h. The HETP values were obtained by dividing the column length (in centimeters) by the number of theoretical plates, Samples. Various known mixtures of trichlorosilane, silicon tetrachloride, trimethylchlorosilane, methyltrichlorosilane, and dimethyldichlorosilane were prepared and used to evaluate the columns. The mixtures were placed in 30-ml serum bottles and closed with serum bottle stoppers to facilitate sample removal and to offset hydrolysis of the silanes. Samples with relatively high siloxane content were prepared n =
(7) E. Glueckauf, Trans. Faraday SOC.,51,34 (1955). VOL. 41, NO. 6, MAY 1969
871
14
Trimothylchlororilano
\
t,Z-Dirndhyl-1 1 3 )-t~tro. shlorodirilor.& ’ Dimolhyldichlororilano
~~~h~~ichlororiI.n.
01
1
21
1 41
1 E1
i 81
1 10 1
1 12 1
1 !I 1
116 1
118 1
20 1 1 22 1
1 24 1
~
RETENTION TIME fminutor)
Figure 2. Chromatogram of methyltrichlorosilane sample on 10% LSX-3-0295 0
2
4
6
8
10 12 14 RETENTION TIME
16
18
20
22
(minuter)
Figure 1. Chromatogram of trichlorosilane (1) containing: ( 2 ) silicon tetrachloride; (3) 1,1,3,3-tetrachlorodisiloxane; (4), 1,1,1,3,3-pentachlorodisiloxane (9,hexachlorodisiloxane; (6) and 1,1,2,3,3-pentaclorotrisiloxane; (7) 1,1,1,2,3,3,-hexachlorotrisiloxane; and (8) 1,1,1,2,3,3,3-heptachlorotrisiloxaneon 10% SF-96
by storage of chloro- or methylchlorosilanes in bottles made of Teflon (Du Pont) with screw caps for several months. Aliquots of 5 p1 were taken with a 50-p1 syringe (Hamilton) which was rinsed with chloroform and dried with nitrogen immediately after injection to prevent the plunger from freezing in the barrel because of rapid hydrolysis of the silanes. Smaller syringes could not be used because the plunger would often freeze before it could be rinsed with chloroform. Retention Times. The temperature of the column was started at 25 “C in each case and programmed at a constant rate to elute the higher boiling components. Retention times for the components in the chlorosilane mixtures are listed in Table -11. The siloxane peaks were identified by trapping the column effluent for analysis by mass spectrometry (8).
RESULTS AND DISCUSSION Separation of the Chlorosilanes and Methylchlorosilanes. The dimethyl-,diethyl- and dibutyl-phthalates were the most effective of the phthalate ester stationary phases in making complete separations of the components. The maximum temperature for these three phases is less than 100 “Cand, consequently, considerable column bleed is evident when the program nears this temperature. Dinonylphthalate has a higher temperature limit, but is not so effective in resolving the methyltrichlorosilane and dimethyldichlorosilane. Dipropyltetrachlorophthalate is even less effective. The silicone oils and gums bleed less at higher temperatures but only those with higher polarity are capable of resolving the chlorosilane mixture. Methyltrichlorosilane and dimethyldichlorosilane elute together when using SF-96, DC704, and DC-710. Silicon tetrachloride and trimethylchlorosilane are not completely resolved with these substrates. The trifluoropropylmethyl silicones, QF-1 oil and LXS-30295 gum, are more polar than the methyl and phenylmethyl silicones and are capable of good resolution of methyltrichlorosilane and dimethyldichlorosilane. A cyanoethylmethyl silicone gum, XE-60, shows the same separation (8) K. R. Burson and C. T. Kenner, J . Chromatographic Sci., 7,63 (1969).
872
e
ANALYTICAL CHEMISTRY
characteristics as the trifluoropropyl silicones due to its polarity. Separation of Siloxanes. The siloxanes have higher boiling points than the chlorosilanes from which they are formed and are eluted at a later time. Because their boiling points increase with increasing chloride substitution, temperature programming is required for elution from the columns that are used for separating the chlorosilanes. The phthalates cannot be used for the siloxane separations because the upper temperature for all of them is below the temperature at which the siloxanes will be eluted from the column. The nonpolar silicone oil SF-96 was found to be the most effective in separating both the chlorosiloxanes and methylchlorosiloxanes. Figure 1 shows a chromatogram of trichlorosilane with a high siloxane content that was separated on this column. The compounds were eluted in order of increasing boiling points when linear temperature programming was used. The boiling points of the siloxanes were determined from their elution times from a graph prepared using normal hydrocarbons of known boiling points (9), and were comparable to literature values (10). Quantitative Analysis of Samples. The purity of trithechloro silane and silicon tetrachloride was determined with SF-96 column. DC-LSX-3-0295 trifluoropropyl silicone gum was found to be the best for analyzing samples of the methylchlorosilanes. Figure 2 shows a chromatogram of a sample of methyltrichlorosilane with impurities of 0.02% silicon tetrachloride, 0.03% methyldichlorosilane, 0.04z trimethylchlorosilane, 0.12% dimethyldichlorosilane, and 0.07x 1,1,3,3-tetrachloro-l,3-dimethyldisiloxane. The concentrations of these impurities were determined by comparison of peak areas with standards prepared by adding known amounts of these impurities to methyltrichlorosilane of 99.99 purity, RECEIVED for review December 23, 1968. Accepted February 19,1969. Presented at the Southwest Regional Meeting of the ACS in Austin, Texas, December 1968. Taken in part from the thesis submitted by K. Ray Burson to the Graduate School of Humanities and Sciences of Southern Methodist University in partial fulfillment of the requirements for the degree of Master of Science. (9) L. E. Green, L. J. Schmaush, and J. C . Worman, ANAL.CHEM., 36, 1512 (1964). (10) V. Bazant, V. Chvalovsky, and J. Rathousky, “Organo-Silicon Compounds,” Publ. House of the Czechoslovak Academy of Sciences, Prague (1965); English Translation, Academic Press, New York (1965).
